This invention relates to impact modified carbonate polymer compositions containing additives which act to reduce the susceptibility of such polymer to burning or at least to minimize the effects of dripping of molten polymer when exposed to fire conditions.
Carbonate polymers derived from reactions of dihydroxyorganic compounds, particularly the dihydric phenols, and carbonic acid derivatives such as phosgene, have found extensive commercial application because of their excellent physical properties. These thermoplastic polymers are suitable for the manufacture of molded parts wherein impact strength, rigidity, toughness, heat resistance, and excellent electrical properties are required.
Unfortunately, however, these polymers exhibit a brief but definite burning time when contacted with an open flame. More importantly, as is often the case, the carbonate polymers contain stabilizers and other additives which are often more combustible than the unmodified carbonate polymer. This is particularly true in the case of impact modified polycarbonates wherein the presence of as little as one percent of a rubbery polymer to improve the impact properties of the blend can lead to substantially increased burn times and dripping of flaming material. As a result, the modified carbonate polymers frequently exhibit substantially poorer resistance to combustion than do the unmodified carbonate polymers.
In attempts to increase the combustion resistance of carbonate polymers, including the impact modified forms thereof, it has been a common practice to incorporate monomeric phosphites, phospheric acid esters, thiophosphoric acid esters containing halogenated alkyl radicals and halogenated organic compounds into the carbonate polymer. However, in order to obtain any noticeable improvement in combustion resistance, these additives have been employed in such large quantities that they often adversely affect many of the desirable physical and mechanical properties of the carbonate polymer.
In U.S. Pat. No. 4,463,130 there were disclosed certain blends of a polycarbonate resin, a styrene polymer and a branched polyphosphate. In addition the composition may include polytetrafluoroethylene. Included within the term "styrene polymer" were styrene graft polymers of rubbery substrates including diene, acrylate, or EPDM (ethylene, propylene, diene monomer) rubbers. Examples 2-5 disclosed blends containing polybutadiene modified polystyrene (HIPS), polycarbonate, polytetrafluoroethylene and additional components.
In U.S. Pat. No. 4,617,345 a polymer mixture comprising an aromatic polycarbonate, a partially hydrogenated block copolymer and a rubbery graft copolymer is disclosed. Fire retarding additives could also be present.
In U.S. Pat. No. 4,299,928 acrylate rubbers were employed in polycarbonate compositions also including fire retarding additives.
In U.S. Pat. No. 4,626,928 fire retardant polycarbonates comprising metal salts of sulfimides, chlorinated or brominated bisphenol polycarbonates and polytetrafluoroethylene were disclosed.